Turbidity Essay

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After the pretreatment process, water is detained for two hours in a basin that allows large turbidity particles to settle at the bottom of the basin. Turbidity is what causes the water to look cloudy due to the colloidal particles, that are 1 to 1000 nm in size, and other suspended particles like sand, silt, organic matter from plants and animals, and microscopic organisms. Turbidity can be a sign of waterborne pathogens present in the water. Turbidity is a primary standard for safe drinking water. The turbidity in water after treatment should be equal to or less than 0.3 nephalometric turbidity unit (NTU) in 95% of the samples per month [1]. This collection of turbidity that falls to the bottom of the basin is called sludge. The sludge is then removed through a sludge removal system periodically. About 85% of turbidity is removed at the presidementation step [1].
The remaining turbidity that is present in the water is too small to settle at the bottom of a basin or tank by gravity. These small particles are negatively charged and repel each other. Before the water can be filtered through membranes, these small particles must first be neutralized by coagulants and coagulant aids [2]. Coagulation and flocculation is a step in the water treatment process in which the small particles are converted into a form that allows them to settle to the bottom of a tank. After the presedimentation step, the water is placed into a rapid mixer along with coagulants and coagulant aids. The coagulants and coagulant aids are chemicals that react with the small turbidity particles and turn them into a precipitate for further exclusion to purify the water. A coagulant is an electrolyte that offers cations to precipitate out the negatively charged c...

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...of the water treatment process. The flocculated water flows into a primary sedimentation basin in which the water is detained up to six hours in dormant conditions. There are two types of primary sedimentation basins. A conventional sedimentation basin is 15 to 20 feet deep to allow proper sedimentation. Water is detained in a conventional sedimentation basin for four to six hours. A high rate sedimentation basin is designed for a high water load that shortens the detention time to one to two hours [3].
Water then flows from the primary sedimentation basins to a final sedimentation basin. At this point the water is clear of turbidity and a small amount of polymer is added to remove any floc that may be present in the water. Chlorine is also added to the water to control biological growth when the water is filtered to remove turbidity that may be present [1,3, 4].

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